The immune response to polysaccharide antigens is highly regulated and has several distinguishing features. Among them are restricted use of immunoglobulin subclasses, restricted use of immunoglobulin variable regions and late development in ontogeny. The basis for selective VH gene usage in response to antigens such as bacterial levan (BL) is poorly understood. BL is a b(2--6) linked polyfructosan with b(2--1) (inulin determinant) linked branch points. We have generated a panel of 107 clonal antibodies (mAb) from BALB cAnN and CBA/CAHN mice (which are closely related at the immunoglobulin locus) following one or two doses of 10 mcg BL. The mAb were examined for isotype, fine specificity, VH and VL usage. mAb from a single injection in both strains are predominantly of the IgM isotype. There is a shift in both strains after 2 injections of BL to a mix of IgM and IgG3 isotypes. There is a major difference in fine specificity between the two strains. All but 2 of the 52 CBA mAb are inulin negative. In BALB/c, 40 of 49 primary mAb are inulin positive; 1 of 6 hyperimmune mAb is inulin positive. VH gene analysis of CBA mAb showed a biased usage of J606 and 36-60 families with an expected frequency of J558 in response to single or multiple injections. InBALB/c mAb there is a pronounced VH restriction to the J606 family (88%) following a single injection of BL. BALB/c gives a more normal, less restricted, response in VH usage to two injections of BL. VL usage is also restricted. There is a correlation in both strains (with only one exception in CBA) between VK11 usage and inulin reactivity of the mAb. There is also a significant overrepresentation of VK10 in primary CBA J558 mAb as well as primary BALB/c X24 mAb. We conclude that CBA and BALB/c mAb specific for BL differ markedly in VH gene family usage and fine specificity and that light chain usage correlates with fine specificity. Molecular studies are in progress to further define the basis for these differences.